1. Field of the Invention
The present invention relates generally to a liquid crystal display device, and more particularly to a liquid crystal display device driven by a lateral electric field and including picture and common electrodes formed on one substrate.
2. Description of the Related Art
Liquid crystal display devices are widely used as display devices for various kinds of equipment such as personal computers, OA equipments, and TV sets because the display devices have many advantages such as lightness, compactness and low power consumption. In recent years, the liquid crystal display device has been also used in mobile terminal equipments such as a mobile phone, a car navigation device and a game player. In the liquid crystal display device, a liquid crystal layer is held between a pair of substrates, respectively having picture electrodes and a counter electrode, and a picture is displayed by selectively passing light from a backlight unit through a modulated liquid crystal.
Recently, obtaining a high quality display characteristic, such as a wide viewing angle, has been desired in the liquid crystal display device. Particularly, a liquid crystal display device using a lateral electric field, including a fringed electric field, attracts attention. In the liquid crystal display devices using lateral electric modes such as an In-Plane Switching (IPS) mode and a Fringe Field Switching (FFS) mode, picture and common electrodes are formed on an array substrate, and molecules of the liquid crystal layer are switched by the lateral electric field.
In the liquid crystal display device, polarizers are arranged so that each of absorption axes on outer surfaces of the array and counter substrates respectively cross to achieve a normally black display mode. For example, when no voltage is applied between the substrates, a black picture is displayed. On the other hand, when a voltage is applied between the substrates, a transmittance gradually increases and a white picture is displayed.
In an active matrix type liquid crystal display device, a thin film transistor (TFT) is used as a switching element. It is well-known that unfavorable “variation of pixel voltage” occurs due to parasitic capacitance between a gate and drain regions of the TFT when the TFT switches from the “on” to the “off” state, that is, when a gate voltage to switch off the TFT is applied to its gate electrode.
In many cases, the variation of the pixel voltage cannot be made equal in all pixels of one picture, and therefore, the variation of the pixel voltage causes display defects such as a flicker phenomenon and an image persistence. In the liquid crystal display device using the lateral electric field, both the picture electrode and common electrodes are formed on the same array substrate. Accordingly, a different parasitic capacitance from that of a liquid crystal display device using a vertical electric mode, in which picture electrode and common electrodes are formed on respective of the substrates, is generated. Therefore, it is difficult to design around the variation of the pixel voltage correctly in consideration of the parasitic capacitance, which results in display defects such as the persistence or the like because of a DC voltage component applied to the liquid crystal layer when a picture is displayed.
In Japanese laid open patent application P2007-183299, a technology to decrease a parasitic capacitance which causes the persistence of the display is disclosed. A pixel electrode is laminated on a common electrode layer through an insulation layer. Consequently, the insulation layer is present between the common and pixel electrodes. The insulation layer functions as a capacitive element which easily causes image persistence. In order to suppress the image persistence, the insulation layer in portions corresponding to electrode apertures is provided with insulating layer apertures.